1. Field of the Invention
The present invention relates to a motor driving apparatus for performing high-speed rotation by phase detection control.
2. Description of the Prior Art
To rotate a motor at a higher speed is one of the most important factors in terms of an improvement in basic performance of the motor, and has been studied and developed for many years. For example, electrical timepieces, which are one kind of products using motors, have recently been provided with multi-functions, and timepieces having various functions such as stop watch functions, alarm functions, and dual-time functions in addition to normal time display functions have been developed and put into practice. In each of these multi-functional timepieces, when the system is initialized in an initial state, e.g. in loading a battery therein, or when the mode is shifted, or the positions of hands are returned to 0 in normal use, the hands must always be fast-forwarded.
A conventional motor driving apparatus will be described below by exemplifying a stepping motor of an electronic timepiece. FIG. 8 is a view showing the arrangement of a motor driving apparatus constituted by a conventional bipolar stepping motor. FIGS. 9 to 14 are plan views each showing the positional relationship between the magnetic poles of a stator and a rotor. As shown in FIG. 8, the bipolar stepping motor serving as a means for converting an electrical signal into a mechanical rotation motion comprises a driving coil 101, a flat stator 102, and a rotor 103. The flat stator 102 has a step 102a. Motor drivers 104a and 104b are arranged to change the potential across the driving coil 101 and flow a current through the driving coil 101, thereby exciting the flat stator 102. In the case of the bipolar motor shown in FIG. 8, when no current flows through the driving coil 101, the pole position of the rotor 103 with respect to the flat stator 102 is a statically stable point 110 shown in FIG. 9. When a current flows through the driving coil 101 to excite the flat stator 102, the pole position of the rotor 103 with respect to the flat stator 102 is an electromagnetically stable point 111 shown in FIG. 10.
Normally in the electronic timepiece, a pulse signal for changing the potential across the driving coil 101 is output from the motor driver 104a or 104b for 4 to 5 mS to flow a pulse current through the driving coil 101, thereby rotating the rotor 103. The rotor 103 rotates during supply of the current to the driving coil 101. When the magnetic pole of the rotor 103 reaches a position shown in FIG. 11 with respect to the flat stator 102, the current flow through the driving coil 101 is stopped, but the rotor 103 inertially rotates to a position in FIG. 12. Then, the rotor 103 damped-oscillates about the statically stable point 110 and finally stops.
When a pulse signal is output from the motor driver 104a after the rotor 103 becomes stationary, to flow a current through the driving coil 101, thereby exciting the flat stator 102 as shown in FIG. 13, the rotor 103 rotates through 180.degree. in a rotational direction A in FIG. 13. When a pulse signal is output from the motor driver 104b opposite to the motor driver which has previously output the pulse signal after the rotor 103 becomes stationary, the rotor 103 further rotates through 180.degree. in the direction A in FIG. 13. The rotor 103 certainly rotates in the direction A in FIG. 13 by flowing a current through the driving coil 101 after the rotor 103 becomes stationary.
When the stepping motor is to be rotated at a high speed, the rotor 103 must be rotated at a high speed, as a matter of course. At this time, an output interval between pulses output from the motor drivers 104a and 104b must be shortened.
If the output interval between pulse signals is shortened so as to rotate the rotor 103 at a higher speed, a next pulse signal must be output even though the damped oscillation of the rotor 103 immediately after rotation has not stopped yet.
If the next pulse signal is output in a state in which the damped-oscillating rotor 103 reaches a position shown in FIG. 14, i.e. the rotor 103 exceeds the electromagnetically stable point 111, the rotor 103 rotates in a direction opposite to the direction A in FIG. 13, i.e. in a direction opposite to a normal direction. Therefore, to stably rotate the rotor 103, the output interval of pulse signals must be set to at least a time required to stabilize the damped oscillation of the rotor 103 upon rotation within a range where the rotor 103 does not exceed the electromagnetically stable point 111.
The total time of the pulse width of a pulse signal and the stabilization time for damped oscillation, i.e. the output period of the pulse signal, is at least about 10 mS. This indicates that the output frequency of the pulse signal is limited to about 100 Hz by the current driving scheme.
The above-described problem, however, has been improved by the scheme disclosed in Japanese Unexamined Patent Publication No. 6-23577 filed by the present applicant. FIG. 15 is a view showing the arrangement of an improved motor driving apparatus according to a prior art, in which a detection coil 105 and a counterelectromotive voltage detection circuit 106 are added to the above-described stepping motor in FIG. 8. The detection coil 105 is wound coaxially with the driving coil 101. The counterelectromotive voltage detection circuit 106 is constituted by a differential amplifier for detecting a counterelectromotive voltage generated in the detection coil 105 upon rotation of the rotor 103. In this prior art, the pole position of the rotating rotor 103 with respect to the flat stator 102 is obtained by detecting a counterelectromotive voltage generated upon rotation of the rotor 103 by the counter-electromotive voltage detection circuit 106 through the detection coil 105, and the output timing of the pulse signal is controlled on the basis of an output from the counterelectromotive voltage detection circuit 106.
In the above-described scheme disclosed in Japanese Patent Laid-Open No. 6-235777, when the pole position of the rotor 103 with respect to the flat stator 102 is to be detected, the detection coil 105 is used as a detection means for detecting a counterelectromotive voltage generated upon rotation of the rotor 103. The conventional stepping motor has only two contact points between the driving coil 105 and an electronic circuit, but the use of the detection coil 105 requires two more contact points between the detection coil 105 and the electronic circuit, i.e. a total of four contact points. The increase in the number of contact points between the coils and the electronic circuit greatly limits the range of structural design such as size and wiring. The use of the detection coil 105 itself increases the coil size, the manufacturing cost, and the like.
As a scheme for detecting a counterelectromotive voltage generated from the rotor 103 without using a detection coil and the like, a scheme disclosed in Japanese Examined Patent Publication No. 61-23516 has been proposed. In this scheme, when a counterelectromotive voltage generated from the rotor 103 is to be detected, one terminal of the driving coil 101 is connected to a power supply potential, and an output from a motor driver at the other terminal is set in a high-impedance state, thereby detecting the voltage level generated in the driving coil. However, since one terminal of the driving coil is connected to the power supply potential in this method, only a signal on the other side with respect to the fixed power supply potential can be detected, and the scheme cannot cope with an AC signal of a counterelectromotive voltage generated from the rotor. Therefore, a timing at which the counterelectromotive voltage goes to 0 level cannot be detected.